Encoding multistate charge order and chirality in endotaxial heterostructures

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS2 due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS2 devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS2/1T-TaS2 heterostructures in which the number of endotaxial metallic H-TaS2 monolayers dictates the number of resistance transitions in 1T-TaS2 lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

[2] Y. Zhao et al., Nat. Commun. 14, 2223(2023).4. In Figure 2g-j insets, the authors presented a schematic diagram of chirality across the heterostructure.Is stacking sequence of the chirality known or just the net chirality.Some clarification would be nice.5.I think there is typo in SI.Shouldn't 'SR830 alock-in amplifier' in page 18 of SI be 'SR830 lock-in amplifier'?

Reviewer #2 (Remarks to the Author):
In their manuscript, Husremovic et.al. present an investigation on the optical, phononic, and electrical properties of 1T-TaS2 crystals subjected to controlled annealing, resulting in the formation of 1H-TaS2/1T-TaS2 heterostructures.The authors thoroughly examine the samples using optical microscopy and establish a connection between changes in optical contrast and the tailored properties of the heterostructure.Multiple advanced characterization techniques are employed, all of which confirm the transformation of the original 1T phase into a layered 1T-1H phase with modified CDW (charge density wave) properties.The findings are intriguing and merit publication; however, similar results and concepts have been previously documented, such as in Nature Communications, 13 (2022) 413 and Nature Communications, 14 (2023) 2223.Consequently, due to the lack of sufficient novelty, I am unable to recommend this manuscript for publication in Nature Communications.

Reviewer #3 (Remarks to the Author):
The manuscript by Samra et al. reports the synthesis of highly tunable endotaxial polytype heterostrucures of 1T-TaS2 and 1H-TaS2 by moderate thermal annealing of nano-thick 1T-TaS2 flakes.By employing electron microscopy, Raman spectroscopy, and electronic transport measurements, Samra et al. systematically studied the effects of the number of endotaxial metallic 1H-TaS2 monolayers on the chiral periodic lattice distortion structure and CDW phase transition behavior of 1T-TaS2 fragments.Furthermore, the authors demonstrate how strain engineering can be used to nucleate the polytype conversions.The research data presented in the manuscript is detailed and logical.Compared to the previous studies, I think this work push the limit of controlling the chirality of 1H/1T-TaS2 heterostructure and also demonstrates some applications.However, I have some specific comments and questions, which are listed below and should be considered by the author before the paper is accepted.1) Firstly, I have some concerns about the novelty of the manuscript.The synthesis of heterochiral 1T-TaS2 vertical heterostructures separated by 1H-TaS2 layers by annealing of 1T-TaS2 thin layers, shown in Figure 1, is not a new discovery, as it has been previously reported by Suk Hyun Sung et al. (Nat Commun 13, 413 (2022)).In addition, the method and theory of polarization dependent Raman measurement to detect the chiralities of CDW in 1T TaS2, shown in Figure 3, hava also been previously reported by H F. Yang et al. (PRL 129, 156401 (2022)).
2) The changes in red optical contrast (ΔOCR) upon annealing are interesting, and this is the first time I have seen such a significant difference in optical contrast between 1T-TaS2 and 1H-TaS2 layers.The manuscript has deeply explored the relationship between ΔOCR and the number of 1H-TaS2 layers, but there is a lack of theoretical explanation for this difference.
3) The manuscript reports that the transition from the 1T phase to the 1H phase was achieved by rapidly cooling to room temperature after annealing at 350 oC for 30 minutes in high vacuum.The annealing temperature here is much lower than the phase transition temperature previously reported by Suk Hyun Sung et al. (720 K, Nat Commun 13, 413 (2022)).The authors should provide some explanations and specific vacuum values.2e displayed an illustration of the α-β superlattice.How do the authors confirm the slip amount of central Ta atoms in the upper and lower Star of David?This determines the supercell size of the heterochirality vertical superstructure, and it raises the question of whether there are other possible configurations of the α-β superlattice.3b and 3c compare the Raman optical activity (ROA) of different regions, but this change does not seem obvious.The authors should provide specific values on the vertical axis for readers to see more clearly.6) Figure 4 explores the effect of 1H-TaS2 content or the number of 1H-TaS2-separated 1T-TaS2 lamellae on the temperature dependence of resistivity.Could the authors provide further discussion on the impact of heterochirality, specifically the relative content of α and β 1T-TaS2 on the phase transition of CDW?

Reviewer 1 comments and responses
In manuscript pdf files, changes in response to the Referee 1 comments have been highlighted

Comment A1:
Authors detail fabrication of verti-lateral endotaxial heterostructure of TaS2 and remarkably illustrate the step-wise transition in both resistance and achirality using electronic measurements and 4D-STEM and correlate them with existence of endotaxial H-TaS2 using high resolution DPC-STEM.Overall, the work is impressive, the manuscript is well presented, and contains substantive novelty for publication in Nature Communications.I think the article is well-suited for the journal and is of great interest to the field of 2D quantum materials.I recommend publication of this manuscript to Nature Communications once following comments are addressed: We thank the reviewer for their positive comments and insightful comments.

Comment A2:
While the results are impressive and the manuscript is well written, I feel sufficient credit to previous works were not given.
For example, Sung et al. [1] is the first work which demonstrated 2D endotaxy of TaS2 and endotaxial polytype heterostructure as a new platform to stabilize latent CDW states.In the context of 2D materials, it introduces the concept of endotaxial design.I suggest the authors to include a sentence stating that the recent synthesis of endotaxial TaS2 offers new mechanisms for control and access of CDW states with a citation that follows on the first paragraph.A. Ribak et al., [3] is another work that highlights T/H polytype interfaces and merits credit.We appreciate the reviewer's comment and acknowledge the importance of giving sufficient credit to previous literature works.To address this comment, we adjusted the original manuscript as follows:

We added the following sentence into the introduction:
To this end, the recent synthesis of endotaxial TaS2 offers new mechanisms for accessing modular CDW systems (Sung, 2022).
2. We cited A. Ribak et al., Sci.Adv. 6 aax9480 (2020) after the following sentence in the introduction: Accordingly, together with flake thickness and doping levels, vertical heterostructuring is a powerful route for engineering CDW transitions.

Comment A3:
Comment A3i: Polytype heterostructure presented in this work is thick slab of 1T-TaS2 separated by H-TaS2.As the authors mentioned in the first paragraph, stabilization of C-CDW in room temperature occurs for thin or monolayers of 1T-TaS2.In this context, shouldn't the 1T-TaS2 presented in this work exhibit NC-CDW phase?For example, in Figure 1c, I would expect to see a mixture of C and NC at least in one of chirality as was shown in Figure S10 of [1], and similarly for resistance vs temperature I expect additional jump in resistance around 200 K.
We thank the reviewer for the constructive comment.We have now examined the possibility of finding coexisting C-CDW and NC-CDW phases in our moderately transformed heterostructures using selected area electron diffraction (SAED).To this end, we examined heterostructures with different degrees of polytype transformations and drew general conclusions summarized as follows: We find that all measured samples comprising ≥ 20% of H-TaS2 manifest an ordered C-CDW phase, characterized by sharp CDW reflections.In contrast, the CDW reflections observed for samples converted to a lesser extent are less well-defined and exhibit peak splitting and angular blurring.The angular blurring suggests the presence of NC-CDW domains, which are rotationally offset from the C-CDW phase by 1.9°.However, our SAED data do not reveal clear hallmarks of the long-range NC-CDW phase, (e.g., 3-fold satellite peaks around the CDW reflections and stronger intensity of higher-order CDW reflections compared to the first order reflections).Thus, we conclude that minimally transformed samples host a disordered C-CDW phase with likely presence of some NC-CDW domains.We added a writeup of this discussion in the main text and added the representative SAED data for heterostructures of different compositions in SI Figure 4.

Addition to Main text, Results:
The ensemble room-temperature ordering of CDW domains in TaS2 heterostructures with different polytype compositions was further probed using SAED.Our findings support that samples comprising ≥ 20% of H-TaS2 manifest an ordered C-CDW phase, characterized by sharp CDW reflections (SI Figure 4b-d).In contrast, the CDW reflections observed in samples with < 20% H-TaS2 appear less well-defined and exhibit peak splitting and angular blurring (SI Figure 4a).Accordingly, we infer that crystals containing less than 20% H-TaS2 content host a disordered C-CDW phase with likely coexistence of some NC-CDW domains.

Addition to Supporting information:
Regarding the absence of the low-temperature upturn in our samples, we note that current flow would primarily take place through the lower-resistance (metallic) H-TaS2 layers at these temperatures.So, transport will be dominated by the H-TaS2.This would preclude the observation of low-temperature upturns in our mixed polytype crystals.

Comment A3ii:
Do you have an explanation for the mechanism of the stabilization of C-CDW in this heterostructure?
In our verti-lateral heterostructures, 1T-TaS2 layers continuously extend across regions with different polytype compositions and corresponding 1T-TaS2 slab thicknesses.We speculate that C-CDW domains in thinner 1T-TaS2 slabs could serve as templates for the nucleation of C-CDW domains in adjacent thicker 1T-TaS2 slabs.However, a comprehensive study on the nano-scale mechanism of C-CDW domain formation in minimally transformed samples lies is beyond the scope of this manuscript.

Comment A4:
In page 9, the authors suggest that the broadening of phase transition is maybe due to strengthened out-ofplane interactions.While this is just a speculation I feel this statement is somewhat contradictory to results regarding heterochirality in Figure 4.In essence, if H-TaS2 screening is enough to disrupt chiral-locking [2], how could it affect the phase transition?
We thank the reviewer for the insightful comment, which is a valid counterpoint to our original hypothesis.Indeed, if we consider that broadening implies defect pinning, an alternative hypothesis for the difference in cooling and warming profiles is that the defect pinning strength is contingent on the CDW phase [c.f.1-4].Defects may exert stronger pinning effects on the CDW domains in the localized C-CDW state compared to the "melted" IC-CDW phase, leading to less well-defined transitions upon warming.We removed the previous proposal and provided this alternative explanation instead in the main text. [

Addition to Main text, Results:
"We note that resistance traces upon warming are noticeably broadened relative to the respective cooling sweeps.This may be understood by considering that strength of defect pinning is contingent on the CDW phase [1][2][3][4].Defects may exert stronger pinning effects on the CDW domains in the localized C-CDW state compared to the "melted" IC-CDW phase, leading to less well-defined transitions upon warming."

Comment A5:
In Figure 2g-j insets, the authors presented a schematic diagram of chirality across the heterostructure.Is stacking sequence of the chirality known or just the net chirality.Some clarification would be nice.
Thank you for pointing out that further clarification is needed.Indeed, we can only determine net chirality from plan-view 4D-STEM.To clarify this point, we added the following sentence into the caption of Figure 2: Addition to Caption of Figure 2: "We note that only the net chirality can be determined, and the exact stacking sequence of the chirality shown here is one of several possibilities as the precise sequence cannot be obtained from plan-view 4D-STEM."

Comment A6:
I think there is typo in SI.Shouldn't 'SR830 alock-in amplifier' in page 18 of SI be 'SR830 lock-in amplifier'?
We thank the reviewer for finding this typo, which is now corrected in the Methods writeup.

Main text, Methods section:
"…with the SR830 lock-in amplifier."

Reviewer 2 comments and responses
In manuscript pdf files, changes in response to Referee 2 comments have been highlighted.

Comment B1:
In their manuscript, Husremovic et.al. present an investigation on the optical, phononic, and electrical properties of 1T-TaS2 crystals subjected to controlled annealing, resulting in the formation of 1H-TaS2/1T-TaS2 heterostructures.The authors thoroughly examine the samples using optical microscopy and establish a connection between changes in optical contrast and the tailored properties of the heterostructure.Multiple advanced characterization techniques are employed, all of which confirm the transformation of the original 1T phase into a layered 1T-1H phase with modified CDW (charge density wave) properties.The findings are intriguing and merit publication; however, similar results and concepts have been previously documented, such as in Nature Communications, 13 (2022) 413 and Nature Communications, 14 (2023) 2223.Consequently, due to the lack of sufficient novelty, I am unable to recommend this manuscript for publication in Nature Communications.
We thank the reviewer for their feedback.However, we note that the central results of this work have not been previously documented.Though, we agree that we could do a better job of emphasizing the novelty of our findings more explicitly in the manuscript.Specifically: 1. Novelty of sample structure.In this study, we demonstrate the preparation of novel polytype heterostructures that consist of interdispersed monolayer H-TaS2 between fewlayer 1T-TaS2 lamellae.This is the opposite of the well-studied heterostructures consisting of monolayer 1T-TaS2 sandwiched within thicker 2H-TaS2 blocks.These are key structural differences that lead to considerable changes in optical and electronic properties (distinct from the endotaxial TaS2 heterostructures studied in any previous work).Notably, moderate heating produces well-defined, coexistent vertical and lateral heterostructures that introduce interesting possibilities for multi-component devices.Such "verti-lateral" polytype heterostructures have not been previously reported on.

Realizing reliable, multi-step switching of resistance and chirality in TaS2
heterostructures.This aforementioned new sample structure presents a distinctive framework that enables the realization of multi-step changes in resistance, structure, and chirality within TaS2 heterostructures, which have not been reported previously.

Addition to Main text, Introduction:
In this work, we demonstrate an approach converse to preceding literatureinterspersing monolayer H-TaS2 between few-layer 1T-TaS2 lamellae-to isolate CDW transitions and realize the a distinctive framework for deterministic engineering of multistate resistance and chirality changes in TaS2 above room temperature.Thus, our work provides a versatile and adaptable roadmap for design of reliable, multilevel, multifunctionality switching in phase change materials.

Addition to Main text, Results:
This unprecedented synchronous switching sets the stage for optoelectronic devices combining charge and chirality degrees of freedom.

Addition to Main text, Discussion:
Moreover, the changes in resistance are accompanied by corresponding changes in chirality, resulting in simultaneous modifications of both optical and electrical properties.

Expanding the degree of realizable chiral states in TaS2 heterostructures.
As a result of points #1 and #2 above we find that these heterostructures display a wide range of potential overall chiralities, setting them apart from previously studied systems like 1T-TaS2 flakes/homointerfaces and heavily transformed 1H-TaS2/1T-TaS2 heterostructures.

Addition to Main text, Results:
Notably, our verti-lateral heterostructures exhibit a broad spectrum of possible overall chiralities, distinguishing them from 1T-TaS2 flakes/homointerfaces and heavily transformed 1H-TaS2/1T-TaS2 heterostructures, which can only manifest a single enantiomorphic state.Specifically, the former are homochiral, comprising fully of α or β, while the latter have been shown to be achiral, hosting an equal proportion of α and β.Accordingly, our verti-lateral heterostructures may enable chiral opto-electronic memory schemes through their wide array of chiral states that can generate strong optical responses at room temperature.
4. New out-of-plane CDW stacking sequence resulting from interlayer slippage.We show that the interlayer arrangement of CDW clusters in polytype heterostructures results in a staggered arrangement of CDW domains that has not been reported in previous work.

Addition to Main text, Results:
The interlayer arrangement of CDW clusters in polytype heterostructures exhibits notable distinctions compared to both 1T-TaS2 flakes and homointerfaces.In pristine 1T-TaS2, CDW clusters can be perfectly eclipsed because their building blocks-Ta ions-are directly aligned in the out-of-plane direction.In contrast, in polytype heterostructures, Ta ions in 1T-TaS2 slabs across 1H-TaS2 interfaces must be laterally offset (Figure 1g-j).Thus, CDW clusters in neighboring 1T-TaS2 slabs must assume a staggered arrangement, resulting in distinct CDW superlattice patterns (2e-f, SI Figure 19).

5.
Elucidating the mechanism of polytype formation and mechanical/strain engineering of designer polytype heterostructures.We demonstrate, to our knowledge for the first time, that H-TaS2 polytype nucleates at macrosocopic flake defects.Further, we develop mechanistic insights into the polytype conversion process with theoretical support and leverage these nanoscale findings to showcase the potential of mechanical and strain engineering for the rational design of vertical and lateral TaS2 heterostructures.Furthermore, our results demonstrate the feasibility of creating intricate, multi-component device architectures by combining substrate patterning with moderate thermal annealing conditions.
6. Development of a convenient optical contrast method for determining polytype composition.Using atomic resolution differential-phase-contrast scanning transmission electron microscopy (DPC-STEM) imaging, we benchmark the optical contrast of polytype heterostructures, greatly simplifying the characterization and tracking of phase transitions in 1T-TaS2 and mixed polytype crystals.

Addition to Main text, Discussion:
The polytype composition of these heterostructures can now be conveniently determined using the optical contrast methodology developed in this work, enhancing the accessibility for characterizing and studying complex TaS2 crystals.

Comment C3:
The changes in red optical contrast (ΔOCR) upon annealing are interesting, and this is the first time I have seen such a significant difference in optical contrast between 1T-TaS2 and 1H-TaS2 layers.The manuscript has deeply explored the relationship between ΔOCR and the number of 1H-TaS2 layers, but there is a lack of theoretical explanation for this difference.
Thank you for bringing this to our attention.Differences in optical contrast between freestanding 2H-TaS2 and 1T-TaS2 have already been documented [1,2].These are related to crystal structure, electronic properties, and dielectric properties.But the discovery that ΔOCR is linearly related to the number of H layers even within mixed 1H/1T endotaxial polytypes is indeed new.In response, we provide some explanation for the optical contrast difference between 1T-TaS2 and H-TaS2 in SI Section 1.2. [

Addition to Supporting information 1.2:
Optical contrast of 2D flakes on SiO2/Si is strongly influenced by their refractive index and absorption coefficient.These intrinsic properties are shaped by the crystal structure and the resulting electronic structure.Thus, the structurally distinct 1T-TaS2 and H-TaS2 flakes on identical substrates exhibit significantly different optical contrasts.We employ this property to quantify the number of H-TaS2 layers formed upon annealing 1T-TaS2 crystals.

Comment C4:
The manuscript reports that the transition from the 1T phase to the 1H phase was achieved by rapidly cooling to room temperature after annealing at 350 °C for 30 minutes in high vacuum.The annealing temperature here is much lower than the phase transition temperature previously reported by Suk Hyun Sung et al. (720 K, Nat Commun 13, 413 (2022)).The authors should provide some explanations and specific vacuum values.
Indeed, the T-to-H transition initiates at approximately 230 °C [1].It is important to note that Sung et al. employed a high temperature annealing process, leading to a strong level of polytype transformation (producing monolayers of 1T-TaS2 separated by thick 2H-TaS2).In contrast, our demonstration is that by operating at substantially lower temperatures, moderate levels of polytype transformation can be achieved to create the vertical-lateral heterostructures discussed in this manuscript.We have included a detailed description of our annealing process in the Methods section. [

Main text, Methods:
The annealing of 1T-TaS2 crystals.The 1T-TaS2 flakes were annealed in high-vacuum (approximately 10 −7 Torr) by rapidly warming to 120 °C at 60 °C /min with a 5 minute hold.This is followed by heating at 11.5 °C /min to 350 °C and a 30 minute hold at 350 °C, before rapidly cooling to room temperature at 13.5 °C /min.We appreciate the reviewer's feedback and acknowledge the need for further explanation in our manuscript.Using differential-phase-contrast scanning transmission electron microscopy (DPC-STEM) imaging, we determined that in polytype heterostructures, Ta ions in 1T-TaS2 slabs are laterally offset across 1H-TaS2 interfaces (Figure 1g-j).Given that Ta ions are responsible for CDW clusters, it follows that clusters in neighboring 1T-TaS2 slabs must assume a staggered arrangement, resulting in distinct CDW superlattice patterns (2e-f, SI Figure 19).We added this information into the main text.

Addition to Main text, Results:
The interlayer arrangement of CDW clusters in polytype heterostructures exhibits notable distinctions compared to both 1T-TaS2 flakes and homointerfaces.In pristine 1T-TaS2, CDW clusters can be perfectly eclipsed because their atomic building blocks-Ta ions-are directly aligned in the out-of-plane direction.Conversely, in polytype heterostructures, Ta ions in 1T-TaS2 slabs across 1H -TaS2 interfaces must be laterally offset (Figure 1g-j).Thus, CDW clusters in neighboring 1T-TaS2 slabs must assume a staggered arrangement, resulting in distinct CDW superlattice patterns (2e-f, SI Figure 19).

Comment C6:
Figures 3b and 3c compare the Raman optical activity (ROA) of different regions, but this change does not seem obvious.The authors should provide specific values on the vertical axis for readers to see more clearly.
We thank you for the comment.

Figure 2e displayed an
Figure 2e displayed an illustration of the α-β superlattice.How do the authors confirm the slip amount of central Ta atoms in the upper and lower Star of David?This determines the supercell size of the heterochirality vertical superstructure, and it raises the question of whether there are other possible configurations of the α-β superlattice.